cli: migrate from gumdrop to clap v3 (#379)

`gumdrop` was originally chosen for its minimalist set of dependencies,
but `clap` v3 has a slimmed down set of dependencies and better UX.

cli/src/bin/yubikey/main.rs

@@ -8,9 +8,9 @@
     unused_qualifications
 )]
 
-use gumdrop::Options;
+use clap::Parser;
 use yubikey_cli::commands::YubiKeyCli;
 
 fn main() {
-    YubiKeyCli::parse_args_default_or_exit().run();
+    YubiKeyCli::parse().run()
 }